An electronic device that utilizes a clock signal to perform its operation may receive the clock signal from an external source. For example, a memory device may receive a clock signal from a host device. In many situations, the external source sends the clock signal during limited periods, such as only when the external source wants to communicate with the device. However, the electronic device may need to utilize a clock signal at times other than the limited periods that the external source is supplying a clock signal
The electronic device may include an internal or on-chip oscillator, such as a crystal oscillator, that generates a clock signal during the periods that a clock signal is not being supplied from the external source. However, an internal oscillator may have certain disadvantages. One disadvantage is that an internal oscillator is costly in terms of the relatively large amount of silicon area that the internal oscillator consumes. Trimming circuitry or a procedure for the internal oscillator may also be needed, further adding to the product cost. Another disadvantage is that when the electronic device switches to using the internal oscillator, the switching may result in time delay as certain circuits, such as phase locked loops (PLL) and delay locked loops (DLL), may require settling time to adjust for the change in clock signal. Added overhead circuitry may be needed to manage this time delay. As such, internal clock generation circuitry that does not consume as much space and that can seamlessly transition to using an internally generated clock signal may be desirable.